Coding unit level transform size restriction for adaptive color transform

ABSTRACT

Aspects of the disclosure provide methods, apparatuses, and non-transitory computer-readable storage mediums for video encoding/decoding. An apparatus includes processing circuitry that decodes prediction information for a current block in a current picture that is a part of a coded video sequence. The prediction information includes an adaptive color transform (ACT) flag indicating whether ACT is enabled for the current block. The processing circuitry determines whether a transform unit (TU) split is performed on the current block based on a maximum transform size that is determined according to whether the ACT flag indicates the ACT is enabled for the current block. The processing circuitry determines a type of the TU split in response to the TU split being determined to be performed on the current block. The processing circuitry performs the TU split on the current block based on the determined type of the TU split.

INCORPORATION BY REFERENCE

This present application is a continuation of U.S. patent applicationSer. No. 17/097,461, “CODING UNIT LEVEL TRANSFORM SIZE RESTRICTION FORADAPTIVE COLOR TRANSFORM” filed on Nov. 13, 2020, which claims thebenefit of priority to U.S. Provisional Application No. 62/988,276, “CULEVEL TRANSFORM SIZE RESTRICTION FOR ADAPTIVE COLOUR TRANSFORM” filed onMar. 11, 2020. The entire disclosures of the prior applications areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example, 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or maybe predicted itself.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEMNVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (105) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar MV derived from MVs of a neighboringarea. That results in the MV found for a given area to be similar or thesame as the MV predicted from the surrounding MVs, and that in turn canbe represented, after entropy coding, in a smaller number of bits thanwhat would be used if coding the MV directly. In some cases, MVprediction can be an example of lossless compression of a signal(namely: the MVs) derived from the original signal (namely: the samplestream). In other cases, MV prediction itself can be lossy, for examplebecause of rounding errors when calculating a predictor from severalsurrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described herein is atechnique henceforth referred to as “spatial merge.”

Referring to FIG. 1C, a current block (111) can include samples thathave been found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (112 through 116, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide apparatuses for videoencoding/decoding. An apparatus includes processing circuitry thatdecodes prediction information for a current block in a current picturethat is a part of a coded video sequence. The prediction informationincludes an adaptive color transform (ACT) flag indicating whether ACTis enabled for the current block. The processing circuitry determineswhether a transform unit (TU) split is performed on the current blockbased on a maximum transform size that is determined according towhether the ACT flag indicates the ACT is enabled for the current block.The processing circuitry determines a type of the TU split in responseto the TU split being determined to be performed on the current block.The processing circuitry performs the TU split on the current blockbased on the determined type of the TU split.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block. The processing circuitry determines that the TUsplit is performed on the current block based on the ACT flag indicatingthat the ACT is enabled for the current block and at least one of awidth and a height of a transform block of the current block beinggreater than the first maximum transform size.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block. The processing circuitry determines that the TUsplit is performed on the current block based on the ACT flag indicatingthat the ACT is disabled for the current block and at least one of awidth and a height of a transform block of the current block beinggreater than the second maximum transform size.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block. The processing circuitry determines the type ofthe TU split as a vertical split based on the ACT flag indicating thatthe ACT is enabled for the current block and a width of a transformblock of the current block being greater than both the first maximumtransform size and a height of the transform block of the current block.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block. The processing circuitry determines the type ofthe TU split as a horizontal split based on the ACT flag indicating thatthe ACT is enabled for the current block and a height of a transformblock of the current block being greater than the first maximumtransform size and greater than or equal to a width of the transformblock of the current block.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block. The processing circuitry determines the type ofthe TU split as a vertical split based on the ACT flag indicating thatthe ACT is disabled for the current block and a width of a transformblock of the current block being greater than both the second maximumtransform size and a height of the transform block of the current block.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block. The processing circuitry determines the type ofthe TU split as a horizontal split based on the ACT flag indicating thatthe ACT is disabled for the current block and a height of a transformblock of the current block being greater than the second maximumtransform size and greater than or equal to a width of the transformblock of the current block.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block, and the first maximum transform size is less thanthe second maximum transform size.

Aspects of the disclosure provide methods for video encoding/decoding,such as any one or a combination of the processes performed by theapparatus. In the method, prediction information for a current block ina current picture that is a part of a coded video sequence is decoded.The prediction information includes an adaptive color transform (ACT)flag indicating whether ACT is enabled for the current block. Whether atransform unit (TU) split is performed on the current block isdetermined based on a maximum transform size that is determinedaccording to whether the ACT flag indicates the ACT is enabled for thecurrent block. A type of the TU split is determined in response to theTU split being determined to be performed on the current block. The TUsplit is performed on the current block based on the determined type ofthe TU split.

Aspects of the disclosure also provide non-transitory computer-readablemediums storing instructions which when executed by a computer for videodecoding cause the computer to perform any one or a combination of themethods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes;

FIG. 1B is an illustration of exemplary intra prediction directions;

FIG. 1C is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example;

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment;

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment;

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment;

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment;

FIG. 8A shows an exemplary encoding flow using a color transform inaccordance with an embodiment;

FIG. 8B shows an exemplary decoding flow using a color transform inaccordance with an embodiment;

FIG. 9 shows an exemplary decoding flow using adaptive color transformin accordance with an embodiment;

FIG. 10 shows an exemplary flowchart in accordance with an embodiment;and

FIG. 11 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Decoder and Encoder Systems

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick, and the like.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, MVs, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values that canbe input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation that the intra prediction unit (452) has generated to theoutput sample information as provided by the scaler/inverse transformunit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by MVs, available to the motion compensation predictionunit (453) in the form of symbols (421) that can have, for example X, Y,and reference picture components. Motion compensation also can includeinterpolation of sample values as fetched from the reference picturememory (457) when sub-sample exact MVs are in use, MV predictionmechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum MV allowed reference area, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4 . Brieflyreferring also to FIG. 4 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415) andthe parser (420) may not be fully implemented in the local decoder(533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture MVs, block shapes, and so on, that may serve as an appropriateprediction reference for the new pictures. The predictor (535) mayoperate on a sample block-by-pixel block basis to find appropriateprediction references. In some cases, as determined by search resultsobtained by the predictor (535), an input picture may have predictionreferences drawn from multiple reference pictures stored in thereference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most one MVand reference index to predict the sample values of each block.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two MVs and reference indices to predict the sample values of eachblock. Similarly, multiple-predictive pictures can use more than tworeference pictures and associated metadata for the reconstruction of asingle block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a MV. The MV points to thereference block in the reference picture, and can have a third dimensionidentifying the reference picture, in case multiple reference picturesare in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first MV that points to a first reference block in the firstreference picture, and a second MV that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quad-tree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the MV is derived from one or more MVpredictors without the benefit of a coded MV component outside thepredictors. In certain other video coding technologies, a MV componentapplicable to the subject block may be present. In an example, the videoencoder (603) includes other components, such as a mode decision module(not shown) to determine the mode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6 .

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, MVs, merge mode information), and calculate inter predictionresults (e.g., prediction block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictionblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard such asHEVC. In an example, the entropy encoder (625) is configured to includethe general control data, the selected prediction information (e.g.,intra prediction information or inter prediction information), theresidue information, and other suitable information in the bitstream.Note that, according to the disclosed subject matter, when coding ablock in the merge submode of either inter mode or bi-prediction mode,there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7 .

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (603), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Coding Tools For RGB Videos

Natural content can be captured in the RGB color space. For a block inthe RGB color space, there can be strong correlation among differentcolor components. Thus, a color space conversion can be used to removethe inter color component redundancy. However, for screen content, manyimage blocks may exist containing different features having highsaturated colors, which lead to less correlation among color components.For those blocks, coding directly in the RGB color space can be moreeffective.

In some related examples such HEVC screen content coding extension(HEVC-SCC), in order to handle different characteristics of image blocksin screen content, an in-loop adaptive color transform (ACT) can beadopted for efficient coding of RGB video content. The ACT can beoperated in a residue domain, and a CU-level flag can be signaled toindicate a usage of the color-space transform.

In an example, the transform matrixes of the ACT can be expressed asfollows:

$\begin{matrix}{{{Forward}{{transform}:\begin{bmatrix}Y \\C_{g} \\C_{o}\end{bmatrix}}} = {{\frac{1}{4}\begin{bmatrix}1 & 2 & 1 \\{- 1} & 2 & {- 1} \\2 & 0 & {- 2}\end{bmatrix}} \times \begin{bmatrix}R \\G \\B\end{bmatrix}}} & \left( {{Eq}.1} \right)\end{matrix}$ $\begin{matrix}{{{Backward}{{transform}:\begin{bmatrix}R \\G \\B\end{bmatrix}}} = {\begin{bmatrix}1 & {- 1} & 1 \\1 & 1 & 0 \\1 & {- 1} & {- 1}\end{bmatrix} \times \begin{bmatrix}Y \\C_{g} \\C_{o}\end{bmatrix}}} & \left( {{Eq}.2} \right)\end{matrix}$

FIG. 8A shows an exemplary encoding flow using a color transformaccording to an embodiment of the disclosure. In FIG. 8A, residuals of acurrent block can be switched to a forward color-space transform module(801). After the forward color-space transform module (801) is applied,there can still be inter-component redundancy that may benefit from anapplication of cross component prediction (CCP). Therefore, a CCP module(802) can be applied to the current block to further improve the codingefficiency. Then a transform module (803), a quantization module (804),and an entropy encoder (805) can be applied to the current block in theencoding flow.

FIG. 8B shows an exemplary decoding flow using a color transformaccording to an embodiment of the disclosure. In FIG. 8B, after beingprocessed by an entropy decoding module (811) and an inversequantization module (812), the encoded bitstream is switched to beprocessed by an inverse transform module (813), an inverse CCP module(814), and an inverse color-space transform module (815), in order togenerate a corresponding residual for the current block.

In some related examples such as VVC, the ACT can be used to enhance thecoding efficiency of video content with a format of 4:4:4. FIG. 9 showsan exemplary decoding flow using the ACT according to an embodiment ofthe disclosure. In FIG. 9 , the color space conversion can be carriedout in the residual domain. Specifically, an inverse ACT can be addedafter an inverse transform module to convert the residuals from YCgCodomain back to the original domain such as RGB domain.

In some related examples such as VVC, unless a maximum transform size issmaller than a width or a height of a CU, a leaf node of the CU can beused as a unit of transform processing. Therefore, the ACT flag can besignaled for the CU to select the color space for coding the residualsof the CU. Additionally, for an inter coded or intra block copy (IBC)coded CU, the ACT can only be enabled when the CU has at least onenon-zero coefficient. For an intra coded CU, the ACT can only be enabledwhen the CU is coded in direct mode (DM), i.e., chroma components of theCU select the same intra prediction mode of luma component of the CU.

In an example, the transform matrixes of the ACT can be described asfollows:

$\begin{matrix}{{{forward}{{transform}:\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix}}} = {{\begin{bmatrix}2 & 1 & 1 \\2 & {- 1} & {- 1} \\0 & {- 2} & 2\end{bmatrix}\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix}}/4}} & \left( {{Eq}.3} \right)\end{matrix}$ $\begin{matrix}{{{inversed}{{transform}:\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix}}} = {\begin{bmatrix}1 & 1 & 0 \\1 & {- 1} & {- 1} \\1 & {- 1} & 1\end{bmatrix}\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix}}} & \left( {{Eq}.4} \right)\end{matrix}$

In addition, to compensate for the dynamic range change of residualssignals before and after the color transform, the quantization parameter(QP) adjustments, for example (−5, −5, −3), can be applied to thetransform residuals.

As shown in Eq. (3) and Eq. (4), the forward and inverse colortransforms may need to access the residuals of all three components.Thus, the ACT can be disabled in two scenarios in which not allresiduals of three components are available. The first scenario isseparate-tree partition, in which luma and chroma samples inside one CTUare partitioned by different structures. Accordingly, the CUs in theluma-tree can only include a luma component and the CUs in thechroma-tree can only include two chroma components. The second scenariois intra sub-partition prediction (ISP), which is only applied to a lumacomponent while chroma signals are coded without splitting. In some ISPrelated examples, except the last sub-partition(s), the othersub-partitions can only include a luma component.

Table 1 shows an exemplary coding unit syntax table including CU levelACT related signaling.

TABLE 1 Descriptor coding_unit( x0, y0, cbWidth, cbHeight, cqtDepth,treeType, modeType ) {  chType = treeType = = DUAL_TREE_CHROMA ? 1 : 0 ...  if( CuPredMode[ chType ][ x0 ][ y0 ] = = MODE_INTRA &&sps_act_enabled_flag &&   treeType = = SINGLE_TREE )  cu_act_enabled_flag ae(v)   ...  } else if( treeType !=DUAL_TREE_CHROMA ) { /* MODE_INTER or MODE_IBC */   if( cu_skip_flag[ x0][ y0 ] = = 0 )    general_merge_flag[ x0 ][ y0 ] ae(v)   ...  }  if(CuPredMode[ chType ][ x0 ][ y0 ] != MODE_INTRA && !pred_mode_plt_flag &&  general_merge_flag[ x0 ][ y0 ] = = 0 )   cu_cbf ae(v)  if( cu_cbf ) {  ...   if( sps_act_enabled_flag && CuPredMode[ chType ][ x0 ][ y0 ] !=MODE_INTRA &&    treeType = = SINGLE_TREE )    cu_act_enabled_flag ae(v)    mts_idx ae(v)   ...  }

In Table 1, when the variable sps_act_enabled_flag equals to 1, the ACTcan be used and the variable cu_act_enabled_flag can be present in thecoding unit syntax. When the variable sps_act_enabled_flag equals to 0,the ACT cannot be used and the variable cu_act_enabled_flag is notpresent in the coding unit syntax. When not present, the variablesps_act_enabled_flag can be inferred to be equal to 0.

When the variable cu_act_enabled_flag equals to 1, the residuals of thecurrent CU are coded in YCgCo color space. When the variablecu_act_enabled_flag equals to 0, the residuals of the current CU arecoded in original color space such as RGB. When not present, thevariable cu_act_enabled_flag can be inferred to be equal to 0.

III. Improvement Of Transform Size Restriction For ACT

In 4:4:4 image coding, the inverse ACT can be used to transform thecolor format from YCgCo format to RGB/YUV format. Before the inverse ACTis performed, all YCgCo three color planes can be decoded in the spatialdomain. That is, three color buffers are needed before performing theinverse ACT. For example, if a size of a CU in the inverse ACT is MxN, atemporary buffer size can be 3×M×N to buffer the decoded residual dataof three planes.

In some related examples, in order to reduce the temporary buffer size,a 64-point transform size restriction is used, in which the variablesps_max_luma_transform_size_64_flag is implicitly set to be false whenthe variable sps_act_enabled_flag is true. Accordingly, when the SPSlevel ACT enable_flag is true for the entire encoded sequence, themaximum transform size can be limited to be 32×32 and the transform sizeof 64×64 is not allowed even for coding blocks which the ACT is notapplied on. In such an example, a TU split is needed when the size ofthe current CU is greater than 32×32, decreasing the coding efficiency.

The present disclosure includes embodiments related to size restrictionof ACT. The size restriction can be used to avoid a large temporarybuffer requirement before performing the ACT transform. Methods areprovided for improving coding efficiency when the SPS level ACTenabled_flag is true for the entire coding sequence. In a method,whether to apply the TU split to a current CU is not only based on themaximum transform size for ACT off (e.g., the variablecu_act_enabled_flag is false), but is also based on whether the ACT isenabled at the CU level for the current CU (e.g., whether the variablecu_act_enabled_flag is true or not) and a predefined maximum transformsize for ACT on (e.g., the variable cu_act_enabled_flag is true).

In one embodiment, for a coding block with ACT on or being enabled atthe CU level (e.g., the variable cu_act_enabled_flag is true), the TUsplit can be performed when a size (e.g., a width or a height) of thecoding block is larger than the predefined maximum transform size forACT on (e.g., the maximum transform size when the variablecu_act_enabled_flag is true). In an example, the predefined maximumtransform size for ACT on can be set as MaxActTbSizeY (e.g., 32). ACT oncan be indicated by the cu_act_enabled_flag or by other ACT information.

In one embodiment, for a coding block with ACT off or being disabled atthe CU level (e.g., the variable cu_act_enabled_flag is false), the TUsplit can be performed when a size (a width or a height) of the codingblock is larger than the maximum transform size for ACT off (e.g., themaximum transform size when the variable cu_act_enabled_flag is false).In an example, the maximum transform size for ACT off can be set asMaxTbSizeY (e.g., 64). ACT off can be indicated by thecu_act_enabled_flag or by other ACT information.

In an embodiment, the predefined maximum transform size for ACT on(e.g., MaxActTbSizeY) can be smaller than the maximum transform size forACT off (e.g., MaxTbSizeY).

In an embodiment, the predefined maximum transform size for ACT on(e.g., MaxActTbSizeY) can be equal to the maximum transform size for ACToff (e.g., MaxTbSizeY).

According to aspects of the disclosure, a type of the TU split can be avertical split or a horizontal split. When at least one of a width and aheight of a transform block of a current block is greater than a maximumtransform size (e.g., MaxActTbSizeY or MaxTbSizeY), the TU split can beapplied to the current block. When the width of the transform block ofthe current block is greater than the height of the transform block ofthe current block, the type of the TU split can be the vertical split.Otherwise, the type of the TU split can be the horizontal split.

Table 2 shows an exemplary transform tree syntax table.

TABLE 2 De- scriptor transform_tree( x0, y0, tbWidth, tbHeight ,treeType, chType ) {  InferTuCbfLuma = 1  if(IntraSubPartitionsSplitType = = ISP_NO_SPLIT &&  !cu_sbt_flag ) {   if((!cu_act_enable_flag && ( tbWidth > MaxTbSizeY || tbHeight >MaxTbSizeY)) ||    (cu_act_enable_flag && (tbWidth > MaxActTbSizeY ||tbHeight > MaxActTbSizeY)) ) {    verSplitFirst = (((!cu_act_enable_flag&& tbWidth >     MaxTbSizeY) || (cu_act_enable_flag &&     tbWidth >MaxActTbSizeY)) &&     tbWidth > tbHeight ) ? 1 : 0    trafoWidth =verSplitFirst ? ( tbWidth / 2) : tbWidth    trafoHeight = !verSplitFirst? ( tbHeight / 2) : tbHeight    transform_tree( x0, y0, trafoWidth,trafoHeight,    treeType, chType )    if( verSplitFirst ) transform_tree( x0 + trafoWidth, y0, trafoWidth, trafoHeight, treeType, chType )    Else  transform_tree( x0, y0 + trafoHeight,trafoWidth, trafoHeight,  treeType, chType )    transform_unit( x0, y0,tbWidth, tbHeight, treeType, 0,    chType )   }  } else if( cu_sbt_flag)

In Table 2, when the variable cu_act_enabled_flag is false (i.e.,!cu_act enabled_flag is true) and at least one of a width and a heightof a transform block of a current CU is greater than the maximumtransform size for ACT off (i.e., tbWidth>MaxTbSizeY∥tbHeight>MaxTbSizeY), or when the variable cu_act_enabled_flag is trueand at least one of the width and the height of the transform block ofthe current CU is greater than the predefined maximum transform size forACT on (i.e., tbWidth>MaxActTbSizeY∥ tbHeight>MaxActTbSizeY), the TUsplit can be applied to the current CU. A type of the TU split can be avertical split or a horizontal split. When the variablecu_act_enabled_flag is false and the width of the transform block of thecurrent CU is greater than both the maximum transform size for ACT offand the height of the transform block of the current CU (!cu_actenable_flag && tbWidth>MaxTbSizeY && tbWidth>tbHeight), or when thevariable cu_act_enabled_flag is true and the width of the transformblock of the current CU is greater than both the predefined maximumtransform size for ACT on and the height of the transform block of thecurrent CU (cu_act enable_flag && tbWidth>MaxActTbSizeY &&tbWidth>tbHeight), the vertical TU split can be applied to the currentCU. Otherwise, the horizontal TU split can be applied to the current CU.

IV. Flowchart

FIG. 10 shows a flow chart outlining an exemplary process (1000)according to an embodiment of the disclosure. In various embodiments,the process (1000) is executed by processing circuitry, such as theprocessing circuitry in the terminal devices (210), (220), (230) and(240), the processing circuitry that performs functions of the videoencoder (303), the processing circuitry that performs functions of thevideo decoder (310), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the intra prediction module (452), the processing circuitrythat performs functions of the video encoder (503), the processingcircuitry that performs functions of the predictor (535), the processingcircuitry that performs functions of the intra encoder (622), theprocessing circuitry that performs functions of the intra decoder (772),and the like. In some embodiments, the process (1000) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1000).

The process (1000) may generally start at step (S1010), where theprocess (1000) decodes prediction information for a current block in acurrent picture that is a part of a coded video sequence. The predictioninformation includes an ACT flag indicating whether ACT is enabled forthe current block. Then, the process (1000) proceeds to step (S1020).

At step (S1020), the process (1000) determines whether a TU split isperformed on the current block based on a maximum transform size that isdetermined according to whether the ACT flag indicates the ACT isenabled for the current block. Then, the process (1000) proceeds to step(S1030). When at least one of a width and a height of the current blockis greater than the maximum transform size, the process (1000)determines to perform the TU split on the current block and proceeds tostep (S1030). Otherwise, the process (1000) determines not to performthe TU split on the current block and terminates.

At step (S1030), the process (1000) determines a type of the TU split inresponse to the TU split being determined to be performed on the currentblock. Then, the process (1000) proceeds to step (S1040).

At step (S1040), the process (1000) performs the TU split on the currentblock based on the determined type of the TU split. Then, the process(1000) terminates.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block, and the process (1000) determines that the TUsplit is performed on the current block based on the ACT flag indicatingthat the ACT is enabled for the current block and at least one of awidth and a height of a transform block of the current block beinggreater than the first maximum transform size.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block, and the process (1000) determines that the TUsplit is performed on the current block based on the ACT flag indicatingthat the ACT is disabled for the current block and at least one of awidth and a height of a transform block of the current block beinggreater than the second maximum transform size.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block, and the process (1000) determines the type of theTU split as a vertical split based on the ACT flag indicating that theACT is enabled for the current block and a width of a transform block ofthe current block being greater than both the first maximum transformsize and a height of the transform block of the current block.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block, and the process (1000) determines the type of theTU split as a horizontal split based on the ACT flag indicating that theACT is enabled for the current block and a height of a transform blockof the current block being greater than the first maximum transform sizeand greater than or equal to a width of the transform block of thecurrent block.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block, and the process (1000) determines the type of theTU split as a vertical split based on the ACT flag indicating that theACT is disabled for the current block and a width of a transform blockof the current block being greater than both the second maximumtransform size and a height of the transform block of the current block.

In an embodiment, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block, and the process (1000) determines the type of theTU split as a horizontal split based on the ACT flag indicating that theACT is disabled for the current block and a height of a transform blockof the current block being greater than the second maximum transformsize and greater than or equal to a width of the transform block of thecurrent block.

In an embodiment, the maximum transform size is a first maximumtransform size based on the ACT flag indicating that the ACT is enabledfor the current block, the maximum transform size is a second maximumtransform size based on the ACT flag indicating that the ACT is disabledfor the current block, and the first maximum transform size is less thanthe second maximum transform size.

V. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 11 shows a computersystem (1100) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 11 for computer system (1100) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1100).

Computer system (1100) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1101), mouse (1102), trackpad (1103), touchscreen (1110), data-glove (not shown), joystick (1105), microphone(1106), scanner (1107), camera (1108).

Computer system (1100) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1110), data-glove (not shown), or joystick (1105), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1109), headphones(not depicted)), visual output devices (such as screens (1110) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted). These visual output devices (such as screens(1110)) can be connected to a system bus (1148) through a graphicsadapter (1150).

Computer system (1100) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1120) with CD/DVD or the like media (1121), thumb-drive (1122),removable hard drive or solid state drive (1123), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1100) can also include a network interface (1154) toone or more communication networks (1155). The one or more communicationnetworks (1155) can for example be wireless, wireline, optical. The oneor more communication networks (1155) can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of the one or more communication networks (1155) includelocal area networks such as Ethernet, wireless LANs, cellular networksto include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wirelesswide area digital networks to include cable TV, satellite TV, andterrestrial broadcast TV, vehicular and industrial to include CANBus,and so forth. Certain networks commonly require external networkinterface adapters that attached to certain general purpose data portsor peripheral buses (1149) (such as, for example USB ports of thecomputer system (1100)); others are commonly integrated into the core ofthe computer system (1100) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1100) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1140) of thecomputer system (1100).

The core (1140) can include one or more Central Processing Units (CPU)(1141), Graphics Processing Units (GPU) (1142), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1143), hardware accelerators for certain tasks (1144), and so forth.These devices, along with Read-only memory (ROM) (1145), Random-accessmemory (1146), internal mass storage (1147) such as internal non-useraccessible hard drives, SSDs, and the like, may be connected through thesystem bus (1148). In some computer systems, the system bus (1148) canbe accessible in the form of one or more physical plugs to enableextensions by additional CPUs, GPU, and the like. The peripheral devicescan be attached either directly to the core's system bus (1148), orthrough a peripheral bus (1149). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1141), GPUs (1142), FPGAs (1143), and accelerators (1144) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1145) or RAM (1146). Transitional data can be also be stored in RAM(1146), whereas permanent data can be stored for example, in theinternal mass storage (1147). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1141), GPU (1142), massstorage (1147), ROM (1145), RAM (1146), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1100), and specifically the core (1140) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1140) that are of non-transitorynature, such as core-internal mass storage (1147) or ROM (1145). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1140). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1140) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1146) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1144)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

APPENDIX A: ACRONYMS

-   AMVP: Advanced Motion Vector Prediction-   ASIC: Application-Specific Integrated Circuit-   ATMVP: Alternative/Advanced Temporal Motion Vector Prediction-   BMS: Benchmark Set-   BV: Block Vector-   CANBus: Controller Area Network Bus-   CB: Coding Block-   CD: Compact Disc-   CPR: Current Picture Referencing-   CPUs: Central Processing Units-   CRT: Cathode Ray Tube-   CTBs: Coding Tree Blocks-   CTUs: Coding Tree Units-   CU: Coding Unit-   DPB: Decoder Picture Buffer-   DVD: Digital Video Disc-   FPGA: Field Programmable Gate Areas-   GOPs: Groups of Pictures-   GPUs: Graphics Processing Units-   GSM: Global System for Mobile communications-   HEVC: High Efficiency Video Coding-   HRD: Hypothetical Reference Decoder-   IBC: Intra Block Copy-   IC: Integrated Circuit-   JEM: Joint Exploration Model-   LAN: Local Area Network-   LCD: Liquid-Crystal Display-   LTE: Long-Term Evolution-   MV: Motion Vector-   OLED: Organic Light-Emitting Diode-   PBs: Prediction Blocks-   PCI: Peripheral Component Interconnect-   PLD: Programmable Logic Device-   PUs: Prediction Units-   RAM: Random Access Memory-   ROM: Read-Only Memory-   SCC: Screen Content Coding-   SEI: Supplementary Enhancement Information-   SNR: Signal Noise Ratio-   SSD: Solid-state Drive-   TUs: Transform Units-   USB: Universal Serial Bus-   VUI: Video Usability Information-   VVC: Versatile Video Coding

What is claimed is:
 1. A method for video decoding in a decoder, themethod comprising: decoding prediction information for a current blockin a current picture that is a part of a coded video sequence, theprediction information indicating whether an adaptive color transform(ACT) is enabled for the current block; in response to the predictioninformation indicating that the ACT is enabled for the current block,performing a transform unit (TU) split on the current block based on oneof a width and a height of the current block being greater than a firstmaximum transform size, the first maximum transform size being less thana second maximum transform size that is determined based on the ACTbeing disabled for the current block.
 2. The method of claim 1, whereinthe performing includes performing a vertical TU split on the currentblock based on the width of the current block being greater than theheight of the current block.
 3. The method of claim 2, wherein theperforming includes splitting the current block into a left block and aright block along the width of the current block, the left block and theright block having a same size.
 4. The method of claim 1, wherein theperforming includes performing a horizontal TU split on the currentblock based on the width of the current block not being greater than theheight of the current block.
 5. The method of claim 4, wherein theperforming includes splitting the current block into a top block and abottom block along the height of the current block, the top block andthe bottom block having a same size.
 6. The method of claim 1, whereinthe prediction information includes a first flag indicating the ACT isused for the coded video sequence and a second flag indicating whetherthe ACT is enabled for the current block.
 7. The method of claim 1,further comprising: in response to the prediction information indicatingthat the ACT is disabled for the current block, performing the TU spliton the current block based on one of the width and the height of thecurrent block being greater than the second maximum transform size. 8.An apparatus, comprising processing circuitry configured to: decodeprediction information for a current block in a current picture that isa part of a coded video sequence, the prediction information indicatingwhether an adaptive color transform (ACT) is enabled for the currentblock; in response to the prediction information indicating that the ACTis enabled for the current block, perform a transform unit (TU) split onthe current block based on one of a width and a height of the currentblock being greater than a first maximum transform size, the firstmaximum transform size being less than a second maximum transform sizethat is determined based on the ACT being disabled for the currentblock.
 9. The apparatus of claim 8, wherein the processing circuitry isfurther configured to: perform a vertical TU split on the current blockbased on the width of the current block being greater than the height ofthe current block.
 10. The apparatus of claim 9, wherein the processingcircuitry is further configured to: split the current block into a leftblock and a right block along the width of the current block, the leftblock and the right block having a same size.
 11. The apparatus of claim8, wherein the processing circuitry is further configured to: perform ahorizontal TU split on the current block based on the width of thecurrent block not being greater than the height of the current block.12. The apparatus of claim 11, wherein the processing circuitry isfurther configured to: split the current block into a top block and abottom block along the height of the current block, the top block andthe bottom block having a same size.
 13. The apparatus of claim 8,wherein the prediction information includes a first flag indicating theACT is used for the coded video sequence and a second flag indicatingwhether the ACT is enabled for the current block.
 14. The apparatus ofclaim 8, wherein the processing circuitry is further configured to: inresponse to the prediction information indicating that the ACT isdisabled for the current block, perform the TU split on the currentblock based on one of the width and the height of the current blockbeing greater than the second maximum transform size.
 15. Anon-transitory computer-readable storage medium storing instructionsexecutable by at least one processor to perform: decoding predictioninformation for a current block in a current picture that is a part of acoded video sequence, the prediction information indicating whether anadaptive color transform (ACT) is enabled for the current block; inresponse to the prediction information indicating that the ACT isenabled for the current block, performing a transform unit (TU) split onthe current block based on one of a width and a height of the currentblock being greater than a first maximum transform size, the firstmaximum transform size being less than a second maximum transform sizethat is determined based on the ACT being disabled for the currentblock.
 16. The non-transitory computer-readable storage medium of claim15, wherein the stored instructions cause the at least one processor toperform: performing a vertical TU split on the current block based onthe width of the current block being greater than the height of thecurrent block.
 17. The non-transitory computer-readable storage mediumof claim 16, wherein the stored instructions cause the at least oneprocessor to perform: splitting the current block into a left block anda right block along the width of the current block, the left block andthe right block having a same size.
 18. The non-transitorycomputer-readable storage medium of claim 15, wherein the storedinstructions cause the at least one processor to perform: performing ahorizontal TU split on the current block based on the width of thecurrent block not being greater than the height of the current block.19. The non-transitory computer-readable storage medium of claim 18,wherein the stored instructions cause the at least one processor toperform: splitting the current block into a top block and a bottom blockalong the height of the current block, the top block and the bottomblock having a same size.
 20. The non-transitory computer-readablestorage medium of claim 15, wherein the prediction information includesa first flag indicating the ACT is used for the coded video sequence anda second flag indicating whether the ACT is enabled for the currentblock.